1. Field of the Invention
The present invention relates to an apparatus for forming a deposited film in which plasma is generated between an electrical power application electrode and a substrate functioning as an electrode arranged opposite to the electrical power application electrode in a vacuum container and a reactive gas introduced into the vacuum container is decomposed to form a deposited film on the substrate.
2. Related Background Art
One of the typical examples of the clean energy sources may be a solar cell. The solar cell is an electronic device which utilizes the photovoltaic effect of converting light energy such as solar energy into electrical energy, and it has lately attracted considerable attention as a part of preventive measures taken in the future against energy problems.
Amorphous silicon has lately attracted notice as a material which can realize a lower-cost solar cell. Amorphous semiconductors, such as amorphous silicon, have occupied attention as materials for use in various types devices, because they can be formed into thin films and be made large in area, their compositional degree of freedom is high, and because their electrical and optical properties can be controlled over a wide range. For amorphous silicon, its optical absorption coefficient is large, compared with silicon crystal, particularly for the light in the vicinity of the peak of solar energy distribution and its film forming temperature is low. Further it has characteristics such that its deposited films can be formed directly from a raw material by using glow discharge and junction formation is easily conducted. Although amorphous silicon has such characteristics as described above and, as for the performance, amorphous silicon having a high conversion factor has already been obtained, it has been desired that its costs should be further reduced. One of the obstacles of realizing lower-cost amorphous silicon may be that its film forming rate in the manufacturing process is low.
In a p-i-n amorphous silicon solar cell produced by the glow-discharge gas decomposition method, a deposited film has been formed in the direction of the film thickness of an i-type semiconductor layer at a fixed film forming rate, for example, at a low rate of 0.1 to 2 Å/sec; therefore, it has taken about 30 minutes to 2 hours to complete the formation of an i-type semiconductor film 4000 Å thick. As one example of the methods of performing high-rate film formation, an attempt has been made to perform film formation utilizing 100% SiH4 gas or 100% Si2B6 gas at a high rate of 5 to 100 Å/sec. Further, in Japanese Patent Publication No. 5-56850, there is disclosed a method in which a film forming rate is increased by decreasing a distance between a power application electrode and a substrate functioning as an electrode.
In the conventional apparatus for forming a deposited film, however, the deformation of the power application electrode may sometimes make it difficult to form a uniform deposited film. Specifically, in order to improve the optical and electrical properties of the deposited film to be formed, the members within the electric discharge chamber need to be heated to a desired temperature, and moreover, their temperature is further increased due to the collision of the particles, such as electrons and ions, accelerated by plasma discharge against the members within the electric discharge chamber. Furthermore, the deposited film is formed on portions other than the substrate, for example, on the power application electrode. As a result, the thermal expansion due to the thermal energy and the stress due to the formation of the deposited film cause deformation of the power application electrode, and hence, generation of non-uniform plasma. This may sometimes make it difficult to form a uniform deposited film.
In Japanese Patent Publication No. 5-73327, there is disclosed an apparatus in which an electric power application electrode is split into a plurality of electrodes and the split electrodes are largely spaced at a large distance and electrically connected to a connection member which allows the distance between adjacent electrodes to be variable. A similar deformation is caused in the substrate, and however the deformation of the substrate can be kept slight by taking preventive measures of, for example, fixing the substrate fast to a substrate holder, or when the substrate is in a strip form, drawing it with a magnet or applying a strong tension to it.
However, when electrically connecting adjacent split electrodes with a connection member, as disclosed in Japanese Patent Publication No. 5-73327, the thickness of the connection plate and the bolts used for connecting the connection plate to the split electrodes affect the distance between the electrode and the substrate as projections, which may cause a disturbance in plasma at such projection-like portions. Furthermore, it is difficult to arrange a plurality of split electrodes in a planar state in one plane simply by connecting the split electrodes with a connection plate, and the decrease of the planeness of split electrodes, in particular in cases where the distance between the electrode and the substrate is small, causes non-uniformity in plasma, which may sometimes give rise to variation in a film forming rate depending on a position on the substrate.
Furthermore, as described above, in the conventional apparatus for forming a deposited film, deposited films are inevitably formed on portions other than the substrate which is an intended portion, such as the power application electrode, because of their configuration. The films formed on the portions other than the substrate which is an intended portion tend to peel, and the films having peeled become the cause of contamination and dust in the subsequent film formation. In order to prevent the quality degradation of the film formed on the substrate due to such contamination and dust, the deposited films formed on the portions other than the substrate need to be removed every time the substrate is replaced, in addition, the power application electrode also needs to be replaced at periodic intervals. This has prevented the continuous production of deposited films and may sometimes prevent the improvement in mass production of the same. In particular, at the time of forming a deposited film with a large area, since the power application electrode becomes large, it takes a lot of time to do such operations as replacing and cleaning the power application electrode frequently, which has been one of the causes of high production costs.